The present invention relates to an encased position measuring instrument for measuring the relative position of two objects comprising a measuring scale, a scanning unit, a housing, a connecting element extending through the housing, and means for sealing the connecting element to the housing.
Position measuring instruments of this type are frequently used on numerically controlled machining equipment for determining the relative position of a workpiece and tool. Generally, the scanning unit is fastened to one of the objects and the measuring scale is fastened to the other of the objects. The attachment is such that relative movement between the two objects results in relative movement between the measuring scale and the scanning unit.
A position measuring instrument of this type is disclosed in a brochure published by Dr. Johannes Heidenhain GmbH, Traunreut, West Germany, entitled "Encased Universal Longitudinal Measuring System ULS, Roll Opening Measuring System WMS", February 1982 edition. The instrument comprises a housing opening for the passage of a connecting rod. A sealing unit seals the opening between the housing and the connecting rod. The sealing unit is fastened to the by means of a plastic tubing extending in the measuring direction. The plastic tubing enables the sealing unit to move in directions perpendicular to the measuring direction, but to not move in the measuring direction. The sealing unit is maintained rigid in the measuring direction by a relatively thick sidewall of the tubing. This, however, creates a disadvantage in that, when changing the position of the housing and sealing unit relative to the connecting rod, constraining forces, perpendicular to the direction of measurement, may be exerted on the connecting rod. These forces may bend the connecting rod and lead to displacement of the scanning unit and measurement errors. Further, uneven radial constraining forces applied by th connecting rod to the sealing unit may result in an incomplete sealing of the housing opening.
It is an object of the present invention to provide an encased position measuring instrument of the above-mentioned type which yields a high degree of measurement accuracy and provides a means of securely sealing the opening between the housing and the connectnng rod.